This invention is an apparatus for sensing the oxygen content in the exhaust gas of an automobile engine and a method of manufacturing such an apparatus. This invention is more particularly related to temperature compensated resistance type sensors.
The exhaust gas of automobile internal combustion engines contains several substances, including carbon monoxide, nitrogen oxides and various unoxidized or only partially oxidized hydrocarbons which contribute to air pollution. These substances can be converted to nonpoisons, like carbon dioxide, nitrogen and water by subjecting the exhaust gas to temperatures in excess of 600.degree. C. while exposing it to catalysts, but, in order to achieve relatively complete conversion, it is necessary that the ratio of fuel to air that is provided to the engine be controlled so that it approximates stoichiometric proportions. Since it is known that the amount of oxygen contained in the exhaust gas provides an indication of whether such stoichiometric conditions exist, it has been suggested that an oxygen sensor be positioned in the exhaust system of an automobile so that it is exposed to its exhaust gas. This sensing element may be connected to a control device which regulates fuel supply and may thereby ensure that the correct ratio of fuel to air is provided to the engine so that the noxious components of the exhaust gas which are emitted into the atmosphere will be at as low a level as is possible.
Such oxygen sensing elements generally fall into two classes. The first of these classes includes solid electrolyte sensors. These sensors are made of a material such as zirconia which responds to the difference in the partial pressures of oxygen between one of its sides which is exposed to exhaust gas and another of its sides which is exposed to ambient air as a reference source. Such solid electrolyte sensors are described in greater detail in U.S. Pat. No. Re. 28,792, reissued April 27, 1976 (previously U.S. Pat. No. 3,400,054).
The other type of oxygen sensing element is generally referred to as a resistance type sensor. These sensors make use of an operative type material such as titania, the electrical resistance of which varies according to the amount of oxygen in the gas to which it is exposed. The principle and operation of such sensors are explained in U.S. Pat. No. 3,558,280 and the use of a titania resistance type sensor in an engine exhaust control is explained in U.S. Pat. No. 3,915,135.
While the resistance type sensors allow for simplicity of construction and installation by virtue of the fact that they do not have to be exposed to both the exhaust gas and a reference source of gas, a problem has been associated with their use. This problem results from the fact that although titania and similar materials are sensitive to changes in the oxygen partial pressure of the gas to which they are exposed, they are also sensitive to changes in temperature. For instance, a typical titania sensor must operate over a range from 300.degree. C. to 900.degree. C. but the electrical resistance of the sensor, over the entire range, does not change in a manner that permits a deliniation between a lean air to fuel mixture and a rich air to fuel mixture. Specifically, for a lean air to fuel mixture over the range of 300.degree. C. and 900.degree. C., the dc resistance of the typical titania sensor drops from 3.times.10.sup.9 ohms down to about 2.times.10.sup.4 ohms. While the dc resistance for a rich air to fuel mixture, over the same range, varies from 3.times.10.sup.6 ohms down to about 40 ohms. Therefore, the resistance characteristics for a rich and a lean mixture for the sensor overlap. Accordingly, for any temperature excursion exceeding about 250.degree. C. it is impossible, with an uncompensated titania sensor, to determine whether the air/fuel ratio is rich or lean. Of course, this is undesirable, as it would not be possible to control the air to fuel mixture.
A number of solutions have been suggested for this problem. It has, for example, been suggested that a heater element in conjunction with a thermostat be provided to maintain resistor type sensors at a constant temperature. Such heater arrangements, however, present certain disadvantages in that they are relatively complex and therefor are subject to failure and are expensive to manufacture. It has also been suggested that a resistor type sensor be made so that the sensor is, itself, temperature compensated. That is, a resistor type sensor is constructed with two resistors. In such a sensor there is provided a first resistor, the resistance of which varies as a function of temperature and the partial pressure of oxygen to which it is exposed, and a second resistor, the resistance of which varies only as a function of temperature. It is known that titania is a suitable material for the first resistor and that a stabilized zirconia material may be used in the second resistor. It is also known that such temperature compensated oxygen sensors may be employed in conjunction with certain circuitry which compares the voltages across the two resistors. This circuitry is thereby able to convert the temperature dependent input signal which it receives from the sensor to a temperature independent output signal by which it controls the air to fuel ratio provided to the engine.
Although such temperature compensated sensors are somewhat simpler than those sensors employing a heater and thermostat, their manufacture may still be unnecessarily complex. This complexity results from the fact that the component resistors of a form of these temperature compensated sensors are in the form of wafers or discs which are connected to electrical leads. If, for example, one of these wafers is composed of titania, titania must first be calcined, cooled, crushed, ball milled with water and then dried. The dry powder must then be blended with solvents, de-aired, cast into tape and dried. The wafers must then be punched from the tape and then suitably positioned on a support. Such a procedure is expensive and time consuming. Furthermore, it entails the risk that the resistor material will become contaminated and it exposes the persons performing it to the health and fire hazards associated with casting solvents.
It is, therefore, the object of the present invention to provide a temperature compensated oxygen sensor which avoids the above mentioned disadvantages of temperature compensated oxygen sensors currently available. Because the temperature compensated oxygen sensor of the present invention does not require the forming and emplacement of a sensor wafer, many of the steps previously associated with the manufacture of temperature compensated sensors are eliminated. Consequently, a durable and less costly sensor may be produced.